THE BONES OF THE TRUNK 



5t 



about forty, but the presternum usually remains permanently separate 

 unless in advanced life, when it may become ankylosed to the mesosternum. 

 The sternal cartilage from which the bone is developed consists origin- 

 ally of two elongated strips, each of which bears the cartilages of nine ribs. 

 The strips are separated for some time by a median fissure, but fusion sub- 

 sequently takes place, and so a single sternal cartilage is formed. The 

 eighth costal cartilage usually loses its connection with the sternum, though 

 it may articulate permanently with the metastemum. The ninth costal 

 cartilage at either side is regarded as dividing into two parts, one of which 



.Epistemal Bone 



..Appears in the 6th Month 

 (intra-uterine) 



^^ I ._— 7th Month 



,8th Month 



9th Month 



ist Year 



/_ 3'd to 6th Year 



Fig. 35A.- 



-ossification of the 

 Sternum. 



Fig. 



35B. — Development of 

 . THE Sternum 

 (Modified from Ruge). 



remains connected with the sternal cartilage and forms with its fellow the 

 metastemum, whilst the other acquires a connection with the eighth costal 

 cartilage. If the parts of the ninth costal cartilages, which remain connected 

 with the sternal cartilage, do not unite with each other over their whole 

 extent, a bifurcated metastemum is the result. They usually, however, unite 

 wholly, or sometimes in such a manner as to leave a foramen at the centre. 

 A sternal fissure is due to the permanent separation of the two original car- 

 tilaginous strips, which, as a rule, unite to form the sternal cartilage. A 

 sternal foramen in the second, third, or fourth segment of the mesosternum 

 is due to ossification from two collateral centres failing to meet at the median 

 line. 



Sometimes two ossicles, called the epistemal bones, are met with at either 

 side of the interclavicular notch of the sternum. These are developed in 

 connection with the suprasternal ligaments, which extend between the inner 

 end of each clavicle and the upper end of the sternum. 



The Thorax as a Whole. 



The thorax constitutes an osseous and cartilaginous cage which 

 lodges the heart and lungs, along with important bloodvessels and 

 nerves, as well as the trachea and oesophagus. It is bounded 

 anteriorly by the sternum, with the costal cartilages and anterior 

 extremities of the first eight or nine ribs ; posteriorly by the bodies 

 of the thoracic vertebrae and the vertebral extremities of the ribs 



